1. Field of the Invention
This invention relates to a lens drive mechanism used in an image-taking optical system or a viewfinder optical system of an image-taking apparatus.
2. Description of the Related Art
A viewfinder optical system of an image-taking apparatus is disposed so that an optical axis of the viewfinder optical system will be substantially parallel to an optical axis of an image-taking optical system. In the driving of lens elements in the viewfinder optical system in the optical axis direction, cam follower portions, which are provided on lens holding members that hold the lens elements in the viewfinder optical system, are made to contact cam groove portions of a cam ring provided in the image-taking optical system, and by converting the rotation of the cam ring to movement in the optical axis direction of the lens holding members in the viewfinder optical system, the field angle of the viewfinder optical system is changed according to a variation of the focal length of the image-taking optical system.
FIG. 4 shows diagrams of a viewfinder drive mechanism disclosed in Patent Document 1.
FIG. 4A is a front view of the viewfinder drive mechanism, FIG. 4B is a side view of the viewfinder drive mechanism, and FIG. 4C is a rear view of the viewfinder drive mechanism.
In FIG. 4, Reference Numeral 101 denotes a first lens, Reference Numeral 102 denotes a first holding frame (lens holding member) that holds the first lens 101, and Reference Numeral 103 denotes a cam follower portion that is provided on the first holding frame 102.
Reference Numeral 107 denotes a second lens, Reference Numeral 108 denotes a second holding frame that holds the second lens 107, and Reference Numeral 109 denotes a cam follower portion that is provided on the second holding frame 108.
Reference Numeral 104 denotes a cam ring that rotates in the direction of the arrow C about an optical axis (image-taking optical axis) L1 of the image-taking optical system. Reference Numeral 104a denotes a first cam groove portion and Reference Numeral 104b denotes a second cam groove portion that are formed on the cam ring 104.
The cam follower portions 103 and 109 are engaged with the first cam groove portion 104a and the second cam groove portion 104b, respectively.
Reference Numeral 105 denotes a guide bar, which guides the holding frames 102 and 108 in the direction of an optical axis (viewfinder optical axis) L2, and Reference Numeral 106 denotes a rotation stopping bar, which prevents the rotation of the holding frames 102 and 108 about the guide bar 105.
On the first holding frame 102, a sleeve portion 102a, which engages with the guide bar 105 in a manner enabling movement in the optical axis L2 direction, and a U-shaped groove portion 102b, which engages with the guide bar 106 and blocks the rotation of the first holding frame 102, are formed.
Likewise, on the second holding frame 108, a sleeve portion 108a, which engages with the guide bar 105 in a manner enabling movement in the optical axis L2 direction, and a U-shaped groove portion 108b, which engages with the guide bar 106 and blocks the rotation of the second holding frame 108, are formed.
Reference Numeral 110 denotes an extension coil spring. The extension coil spring 110 is spanned across the first holding frame 102 and the second holding frame 108 in a state in which it forms a predetermined angle with respect to the guide bar 105.
The first holding frame 102 is thereby energized in the direction of the arrow A in FIG. 4B, the second holding frame 108 is energized in the direction of the arrow B in FIG. 4B, the cam follower 103 is pressed against a side wall of the cam groove portion 104a, and the cam follower 109 is pressed against a side wall of the cam groove portion 104b. 
Also, since the extension coil spring 110 is positioned in a state in which it forms a predetermined angle with respect to the guide bar 105, component forces F1 and F2 (see FIG. 4A and FIG. 4C) are generated in directions substantially orthogonal to the optical axis L2 direction, and by these component forces, the sleeve portions 102a and 108a of the lens holding frames 102 and 108 are pressed against the guide bar 105 and eliminated of play. Vibrations of the lenses 101 and 107 during movement are thereby lessened to restrain vibrations of an image.
When in this state, the cam ring 104 rotates in the direction of the arrow C in the FIG. 4, the cam follower portions 103 and 109 are made, by the energizing force of the extension coil spring 110, to move without play along the cam groove portions 104a and 104b and the holding frames 102 and 108 moves in the optical axis L2 direction.
FIG. 5 shows diagrams of a viewfinder drive mechanism disclosed in Patent Document 2.
FIG. 5A is a sectional view resulting from sectioning the viewfinder drive mechanism along a plane containing an optical axis L2 of a viewfinder optical system, and FIG. 5B is a sectional view resulting from sectioning the viewfinder drive mechanism along a plane orthogonal to the optical axis L2.
A first movable lens unit 210 and a second movable lens unit 211 are lens units (each including lens and a lens holding member), which move when the zoom setting is changed in a variable zoom viewfinder optical system. Lenses 209 and 212 are fixed to a viewfinder block 208. Lens units 210 and 211 are engaged, in a manner enabling movement in the direction of the optical axis L2, to a rectilinear guide shaft 213, which is fixed to the viewfinder block 208.
The first movable lens unit 210 and the second movable lens unit 211 have rotation stopping bosses 210a and 211a, respectively. The rotation stopping bosses 210a and 211a engage with groove portions, which are formed on the viewfinder block 208 and extend in the optical axis L2 direction, to prevent the rotation of the lens units 210 and 211 about the axis of the rectilinear guide shaft 213.
The first movable lens unit 210 and the second movable lens unit 211 have cam follower bosses 210b and 211b as follower pins and respectively have spring latch bosses 211c (the spring latch boss of the first movable lens unit 210 is unillustrated). The cam follower bosses 210b and 211b are put in contact with cam groove portions 214a and 214b of a cylindrical cam 214 by the energizing force of a movable lens energizing spring 219 that is latched onto the spring latch bosses 211C formed on the lens units 210 and 211.
The cylindrical cam 214 is a part of a viewfinder zooming mechanism that rotates via a rotation shaft 215 and is held by the viewfinder block 208. The cylindrical cam 214 is set in the position in the optical axis L2 direction by receiving the energizing force of a cylindrical cam energizing spring 216.
The cylindrical cam 214 also has a second gear 214c as a follower gear, which engages with a first gear 217b, provided on an arcuate cam 217, and rotates by receiving the driving force from the arcuate cam 217.
Furthermore, the cylindrical cam 214 has, as viewfinder lens drive cams, cam groove portions 214a and 214b, which are provided in spiral form. By the rotation of cylindrical cam 214, the first movable lens unit 210 and second movable lens unit 211 are driven in the optical axis L2 direction via the cam follower bosses 210b and 211b that are in contact with the cam groove portions 214a and 214b. 
The arcuate cam 217 is a principal portion of a coupling mechanism that couples the cylindrical cam 214, which is the part the viewfinder zooming mechanism of the variable zoom viewfinder optical system, according to the changing of the image-taking magnification of the image-taking lens barrel. The arcuate cam 217 is disposed to be movable within a predetermined range along an outer circumference of a fixed barrel 201 and is set in position in the optical axis L1 direction by flange portions 201b and 201c that are provided on the fixed barrel 201.
Also, the arcuate cam 217 has a cam groove portion 217a, a drive pin 218 serving as a drive portion is engaged with the cam groove portion 217a, and the cam groove portion 217a serving as a follower portion receives a driving force from the drive pin 218. The arcuate cam 217 is furthermore equipped with the first gear 217b serving as a drive gear portion that engages with the second gear 214c provided on the cylindrical cam 214. And when the image-taking lens barrel performs a zooming operation and the drive pin 218 moves in the optical axis L1 direction, the cam groove portion 217a is pushed and the arcuate cam 217 is thereby moved along the outer circumference of the fixed barrel 201. By this operation of the arcuate cam 217, driving force is transmitted to the cylindrical cam 214.    [Patent Document 1] Japanese Patent Application Laid-Open No. 2000-75187    [Patent Document 2] Japanese Patent Application Laid-Open No. 2001-242507
However, with each of the above-described conventional viewfinder drive mechanisms, since two lens units are engaged to a single guard bar, the length of a sleeve portion that is formed integral to each lens unit cannot be made adequately long. There is thus play, etc., between the lens units and the guide bar and the lens units cannot be moved in a stable manner in the optical axis direction.